A common imaging modality, in many parts of the electromagnetic spectrum, entails illuminating an object with an illuminating beam and then detecting radiation that is scattered by the object. Flash photography is an example. Over the course of the last 20 years, x-ray backscatter imaging, for example, has become a well-established means of carrying out one-sided x-ray inspection of vehicles, cargo containers, baggage, and personnel. Because organic contraband such as drugs and explosives is characterized by a relatively low atomic number, it is effective at scattering x-rays and these materials therefore show up as bright, easily visible regions in a backscatter image.
Target discrimination and the enhancement of target return signal over foreground scatter noise has long been a challenging issue in active remote sensing modalities such as radar or lidar. Local scatter rejection is particularly vexing in the domain of x-ray scatter in that polarization-selecting strategies that are employed in radar and lidar to enhance target return relative to foreground scatter are unavailing in x-ray scatter applications where the Compton scattering mechanism from bound electrons is polarization non-preserving.
Jaffe, Underwater Optical Imaging: The Design of Optimal Systems, Oceanography, pp. 40-41 (November, 1988) discusses a common instance of imaging through a scattering medium, namely underwater photography. In that context, scatter avoidance by lateral offset of the illuminating beam and the camera is believed to be effective only at distances up to 2-3 attenuation lengths, i.e., where the scatter return by the imaged object itself is attenuated by the intervening water by no more than about e3≈20. In x-ray backscatter applications, not only can x-ray attenuation at lower energies be substantially greater, but, additionally, at all energies, the cross section for scatter by atoms of the intervening air is comparable to that of the distant target itself, since atoms of substantially comparable atomic numbers are involved.
Time-gating has been suggested for discrimination of distant x-ray targets, as in U.S. Pat. No. 7,505,562 (Dinca), incorporated herein by reference, where time-resolution capabilities are deemed of particular advantage in long-range applications that are noise-limited by air scatter. Time-gating, however, might be rendered impractical due to temporal constraints of source and/or detector response.
The use of the differential étendue (the product of detector area times the solid angle a detector subtends relative to a source) presented by offset backscatter detectors has been suggested “in order to optimize the efficiency of a system in discriminating among x-rays scatter from various selected regions of the space penetrated by a probe beam,” in U.S. Pat. No. 6,424,695 (Grodzins, the “'695 patent), incorporated herein by reference.
FIG. 1, reproduced from the '695 patent, shows laterally offset detectors in the context of x-ray backscatter imaging. A beam 10 of penetrating radiation is incident upon one or more objects 12 and 20 which may be concealed from view, such as by surface 30 which may be the surface of a wall or may be a surface of an enclosure or container 14. A volume 2 posterior to surface 30 or contained within enclosure 14 may be referred to, herein, without limitation, simply as “enclosure 14.” “Penetrating radiation” refers to electromagnetic radiation of an appropriate range of energy and intensity as to penetrate container 14 and objects 12 and 20, and will be referred to, without limitation, in the following description as x-ray radiation. Beam 10 will similarly be referred to, without limitation, as an x-ray beam. Beam 10 is generated by a source (not shown) of penetrating radiation which may, for example, be an x-ray tube or a radioactive source. Plane 30 tangential to a point at which beam 10 penetrates surface enclosure 14 is referred to as the “plane of incidence.”
X-rays 10 are scattered by objects 12 and 20, giving rise, for example, to scattered x-ray paths 16, 18, 22, 24, and 26. Backscatter detectors 3, 4, 5, and 6 are disposed on the same side of container 14 as source 46, with detectors 3 and 5 on one side of beam 10 and detectors 4 and 6 on the opposite side of the beam. X-rays 10 are preferably in the form of a pencil beam that is raster scanned in the plane perpendicular to the line of the detectors The Grodzins '695 patent taught that the position and relative sizes of backscatter detectors 3, 4, 5, and 6 may be chosen to optimize the efficiency of the system in discriminating among x-rays scattered from various selected regions of the space penetrated by beam 10, and to obtain images that enhance scattering features located at different depths into container 14. Radiation scattered from more distant scattering sources such as object 20 is detected preferentially by exterior detectors 5 and 6 relative to interior detectors 3 and 4 since the detected flux is substantially proportional to the solid angles (depicted in projection in the plane of the paper) designated respectively as Ω6far and Ω3far, subtended by the respective detectors. The collection area of exterior detectors 5 and 6 may be increased relative to the collection area of the interior detectors 3 and 4 in order to enhance the magnitude of Ω6far relative to Ω3far for the more distant scattering sources 20. By way of contrast, for nearer object 12, the ratio of solid angles (depicted in projection in the plane of the paper) designated respectively as Ω6near and Ω3near, subtended by exterior detectors 5 and 6 relative to interior detectors 3 and 4, favors detection by the interior detectors. The Grodzins '695 patent nowhere suggests that discrimination between a more distant scattering source and a less distant scattering source might apply to intervening air scatter.
U.S. Pat. No. 7,551,715 (Rothschild, the “'715 patent”), incorporated herein by reference, teaches “separating the location of the x-ray detectors from the x-ray source.” In the case of the '715 patent, it is taught that the detectors are closer to the scattering object than the rest of the imaging system, allowing for more flux scattered by the target and less flux scattered by intervening air to be collected than if the detectors were co-located with the x-ray source and other equipment. However, the '715 patent teaches that, in order to discriminate the return signal from a target relative to air scatter, detectors are to be placed at a distance from the source in the direction toward the target. This is described in col. 14, lines 34ff of the '715 patent, referring to the configuration of detectors shown in FIG. 2 (FIG. 23 of the '715 patent).
Referring to FIG. 2, which appears in the '715 patent as FIG. 23, detectors 140 near to the primary beam 200 receive an air scatter signal which will ‘fog’ the image. The noise caused by this effect is mitigated, according to the '715 patent, by reading the signals from detectors 140 at different distances in separate channels 74, so that person 76 is imaged in only one detector, and that image contains only the air scatter background from that detector (or the set of detectors in that region). Detectors 140 located along the path of the primary beam 200 that are not near to the target receive much less signal from the target and therefore have a much higher ratio of noise to signal than detectors located near to the target. The overall signal-to-noise ratio may be improved by ignoring the entire signal from detector which are not near the target.
However, operationally, it may be desirable to use detectors which are remote from the target, and which are located at substantially the same distance from a target as the source 46 of the primary beam. No solution to this problem has ever been suggested.